The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Heat exchange technology is a ubiquitous part of many chemical and industrial processes. Process streams generally need heat adjustment to meet requirements for a unit process, transport, discharge or the like. Utilisation of the different temperatures of process streams within industrial processes using, for example pinch analysis methodology, can assist in minimising energy consumption of chemical processes. In many cases, excess heat from one or more process streams can be transferred to a cooler process stream through a heat exchanger.
Purified process water and/or other liquids can also be required in a large number of chemical and industrial processes. Purified water can be produced using a number of unit operations, such as distillation, osmosis, membrane filtration or the like. All these processes require a significant amount of energy to operate.
One process that can be used to produce purified process water is membrane distillation. Membrane distillation is a separation method in which a porous membrane is used to separate a vapour phase from a hot vapourising liquid feed (or retentant) on one side of the membrane and feed that vapour to a cold condensing, permeate fluid, or in some cases a cold surface, on the other side. A vapour pressure temperature difference is established across the membranes sides to create a vapour pressure difference between the membrane sides which is the driving force for the diffusion. Temperature difference across the membrane can conveniently create the vapour pressure difference, but vapour can also be drawn away from the membrane by other means. Separation is achieved utilising the relative volatility of various components in the vapourising fluid enabling vapour from components in the vapourising fluid of higher volatility to passes through the membrane pores by a convective or diffusive mechanism. In the case of membranes with smaller pores, the membrane can also act selectively by molecular sieving and/or adsorption based separation. This is more commonly referred to as pervaporation. In any case, the vapour penetrates through the porous membrane, and condenses in the cooler fluid or surface on the condensing permeate side or in an externally mounted condenser unit. The condensed vapour is therefore removed from the retentant and thus is beneficial in its effect to separate the vapour from the retentant. An example of such benefit is the desalination of salty water.
There are four broad types of membrane distillation systems:
1. Direct contact membrane distillation (DCMD), where both the warm, vaporising vapourising stream and the cold condensing stream (distillate stream) are in direct contact with the membrane.
2. Air gap membrane distillation (AGMD), where a condenser surface contacting the condensing stream is separated from the membrane by an air gap.
3. Sweeping gas membrane distillation (SGMD), where the vaporised portion of the vapourising stream is removed in vapour form by an inert gas.
4. Vacuum membrane distillation (VMD), where the vaporised portion of the vapourising stream is removed in vapour form by vacuum.
Each of these membrane distillation systems require an external heat source and sink to heat the vapourising fluid and cool the condensing fluid. The vapourising fluid is typically heated using waste or fuel derived (combustion) sources. The condensing fluid is typically cooled using a cooling vent, such as a convective air cooler.
Energy optimisation of membrane distillation systems have generally concentrated on the use of cheap or waste heat sources, for example solar and waste process heat, to heat and vapourise the liquid feed of these systems.
For example, Japanese patent publication JP62057611A describes an air gap membrane distillation system for desalinating seawater using diesel engine waste heat from a cooling fluid of that engine. This desalination system comprises two non-permeable condensing heat-transfer plates provided on the outside of two permeable membranes, forming a liquid condensate passage therebetween, and two non-permeable heating heat-transfer plates located between both permeable membranes, forming a heat source fluid passage therebetween. Furthermore, a raw liquid passage is formed between the heating heat-transfer plate and the permeable membrane. Cooling fluid from a diesel engine is passed through the heat source fluid passage as the heat source fluid. Seawater is fed into a cooling liquid passage to cool the heat-transfer plate and the resulting heated seawater is sent into the raw liquid passage for distillation through the membrane distillation system. Energy efficiency in this system is obtained by capturing lost heat from the membrane distillation process in the incoming seawater prior to being fed into to the membrane distillation membrane.
United States patent publication US2010/0072135A1 describes a membrane distillation in which a distillate is created by passing the heat of condensation (latent heat) towards a condenser surface which is contact with the feed stream of the membrane distillation system, enabling at least part of the latent heat to be transferred to that feed stream. Energy efficiency in this system is again obtained by capturing lost heat from the membrane distillation process in the incoming feed stream prior to being fed into to the membrane distillation membrane.
Furthermore, the paper “Membrane distillation and applications for water purification in thermal cogeneration plants” by Alaa Kullab and Andrew Martin, Separation and Purification Technology 76 (2011) 231-237 (“Kullab and Martin”), describes a cogeneration type membrane distillation (MD) process which uses waste heat produced from a first industry process (district heating supply) to supply heat to an unrelated industrial process (municipal water, used as cooling water for the MD process). The test unit produces 1 to 2 m3/day of purified water. In this case, the MD unit is being utilised in a cogeneration configuration, where waste heat from a first industrial process is useful elsewhere in another industrial process. Internal process energy efficiency is therefore not achieved in each individual industrial process.
None of these prior membrane systems assist in heat optimisation of process streams within a proximate and related chemical process in that plant, and more particularly utilise internal heat recycling for an industrial process or plant. Energy optimisation of the systems focuses on energy efficiency of the membrane distillation process in isolation to the overall chemical process in which that membrane distillation system is a part of.
It would therefore be desirable to provide a membrane distillation system which can provide a more energy efficient means of heating and cooling the process streams of a co-located chemical or industrial process in an industrial plant, and more particularly heating and cooling the process streams utilising internal heat recycling for a plant.